Pressure; Function Of A Safety Valve - Leser 411 Instructions De Service

Safety valves have a name plate showing the
following data:
•
Date of order
•
Technical data
•
Test pressure
•
VdTÜV type test approval number
•
CE-marking and identification number of
E
the notified body.
For safety valves without type test approval,
only order data and technical data are included.
Further marks required are either moulded into
the casting, or, for safety valves with threaded
connections, punched in. Safety valves with a
heating jacket have a separate name plate for
the heating jacket.
If technical changes are made, always check
whether the identification has to be adjusted.
Modifications on valves or identifications may
only be carried out by trained personnel (refer
to section 12.14).
4

Pressure

Definitions:
a) Test pressure: the pressure that the safety
valve is set to at LESER's. The outlet side
of the safety valve is open to atmospheric
pressure.
b) Set pressure: the predetermined pressure
at which a safety valve under operating
conditions commences to open.
c) Opening pressure: the pressure at which
the safety valve discharges the certified
flowing capacity (this may also be given as
the difference from the set pressure in
per cent
d) Reseating pressure: the value of the inlet
static pressure at whicht the disc re-
establishes contact with the seat or at
which the lift becomes zero.
e) Operating pressure: the pressure at which
the plant operates.
f) Built-up
existing at the outlet of a safety valve
caused by flow through the valve and the
discharge system.
g) Superimposed back pressure: the pressure
existing at the outlet of a safety valve at the
time when the device is required to operate.
h) Back pressure: the total of built up and
superimposed back pressure.
Pressure shall be stated as overpressure [bar g
or psig] above atmospheric pressure.
Unless stated otherwise, the pressure specified
by the customer is set with atmospheric
pressure acting on the outlet side (test pressure
= set pressure).
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opening pressure difference).
back pressure: the pressure
If there is pressure on the outlet side
(superimposed back pressure), a force is
produced, which acts on the rear side of the
disc. This increases the set pressure by exactly
the value of this pressure. If the superimposed
back pressure is constant, it is possible to
adjust the differential pressure by reducing the
value of the test pressure by the value of the
back pressure (test pressure
If there is no superimposed back pressure, the
set pressure will drop. The back pressure
intended must not be exceeded because then it
would also exceed the set pressure.
The maximum pressure that a safety valve may
be operated at regardless of the test pressure
depends on a
Among these are:
•
Materials
•
Medium temperature
•
Design pressure
•
Flange classes
•
Others.
These should be taken into consideration when
selecting a safety valve.
The value of the operating pressure must
permanently be lower than the reseating
pressure difference plus 5%. Otherwise LESER
cannot guarantee that the valve will close
securely after opening (exception: if the valve is
fitted with a supplementary loading system,
refer to section 15).
5

Function of a safety valve

A performance certificate is required to ensure
that the required mass flow is discharged by the
safety valve if necessary.
Pipes leading to the safety valve have to be
fitted so that large hydrodynamic losses are
prevented. The edges at the pipe inlet should
be at least chamfered, but preferably rounded.
The notes on dimensioning given in the
regulations, standards
information sheets must be adhered to.
Safety valves may only be disabled by means
of shut-off valves provided that the pressurised
equipment is protected against excessive
pressure by other safety devices or that the
plant or equipment is shut down altogether.
Safety valves are guaranteed to work perfectly,
if the built-up back pressure on the outlet side
does not exceed 15% of the test pressure
minus the superimposed back pressure (if
available).
Built-up and superimposed back pressures may
be compensated by an amount not exceeding
set pressure).
number of factors.
and manufacturer's
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